Metal treatment – Compositions – Heat treating
Patent
1980-10-27
1982-07-13
Skiff, Peter K.
Metal treatment
Compositions
Heat treating
148 12F, 148 36, C21D 804, C21D 948
Patent
active
043392849
DESCRIPTION:
BRIEF SUMMARY
Technical Field
The present invention relates to a method of producing non-ageing cold rolled steel sheets, and especially relates to a method of producing non-ageing cold rolled steel sheets having a remarkably excellent deep drawing property.
Background Art
Cold rolled steel sheets obtained by subjecting a rimmed steel or aluminum killed steel to decarburization and denitrogenization annealing in a box type open coil annealing furnace have a remarkably excellent deep drawing property, but the annealing cost is high and further cold rolled steel sheets having poor ageing resistance are sometimes produced due to incomplete decarburization and denitrogenization.
It has been well known that the carbon content of steel must be thoroughly reduced in order to improve the deep drawing property, that is, r value, of cold rolled steel sheet without carrying out decarburization and denitrogenization annealing. However, when a cold rolled steel sheet having a satisfactorily low carbon content of, for example, not higher than 0.02% is annealed, the following drawbacks (a), (b) and (c) occur. very small, it is impossible to fix solute carbon as carbide form by making the solute carbon precipitated as carbide during cooling step of box annealing. Therefore, a large amount of solute carbon remains in the annealed steel sheet, and when the annealed steel sheet is left to stand for a long period of time before the steel sheet is pressed, the steel sheet ages at room temperature. content, and what is worse, the ferrite matrix undergoes a change, so as to have a large ferrite crystal grain size after the box annealing. So, tensile strength is lowered, and wall break is caused during the drawing. ferrite crystal.
As described above, there are various drawbacks in the conventional method, wherein an extra low-carbon steel having merely a carbon content of not more than 0.02% is used to improve the elongation and the r value.
In order to obviate these drawbacks, a non-aging low-carbon steel containing a small amount of niobium, which serves to fix solute carbon and to form fine crystal grains, and a method of producing it have been proposed in Japanese Patent Application Publication No. 35,002/78 claiming priority based on U.S. Pat. No. 3,761,324. According to this disclosure, it is necessary that at least 0.025% of uncombined niobium, that is, niobium which is not fixed by carbon, remains in the low-carbon steel.
However, the steel obtained by the above described method has a high r value of at least 1.8, but has a low elongation of not higher than 48% as compared with the elongation of 50-54% in the ordinary decarburized and denitrided steel. As the result, the low-carbon steel sheet obtained by the above described method has a non-ageing property, but has a poor deep drawing property. Moreover, the steel has a drawback that a large amount of an expensive alloy metal of niobium must be used in the production thereof.
The object of the present invention is to provide a method of producing non-ageing cold rolled steel sheets having a remarkably excellent deep drawing property which are free from the above described drawbacks in conventional method.
Disclosure of the Invention
The inventors have newly found out that it is effective to add niobium to an extra low-carbon steel in an amount less than the necessary amount for fixing completely carbon to convert partly the carbon into NbC, to precipitate the remaining carbon on the nuclei of the above described NbC during the cooling step of box annealing, and to utilize the effect of niobium for suppressing the grain growth of ferrite, and has accomplished the present invention.
That is, the feature of the present invention consists in a method of producing non-ageing cold rolled steel sheets having a remarkably excellent deep drawing property, comprising preparing a steel consisting of, in % by weight, not more than 0.01% of carbon, not more than 0.2% of silicon, 0.05-0.40% of manganese, not more than 0.02% of phosphorus, not more than 0.02% of sulfur, not more than 0.01% o
REFERENCES:
patent: 3761324 (1973-09-01), Elias et al.
patent: 3865637 (1975-02-01), Matsudo et al.
Hashimoto Osamu
Sato Susumu
Tanaka Tomoo
Kawasaki Steel Corporation
Skiff Peter K.
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